On airliners the probes seem to be heated during the whole flight. This makes me ask myself different but related questions:

Accidents occurred due to probes freezing. How can a heated pitot probe freeze?

Extending the life of the tube seems a questionable choice, I'd rather expect the technology to be improved and be able to sustain permanent heat. Heating a small metal device from -56°C to 5°C (or maybe more, say 100°C, taking into account comments) looks like feasible (not taking into account air friction) ⇢ The problem seems to be how to transfer enough heat from the metal (which is very hot) to the ice accretion.

A short-circuit is easily terminated by a fuse or a circuit breaker. Why does the crew have to worry about switching the heat manually, adding a possibility to have it OFF unintentionally?

$\begingroup$I don't know what temperature pitot tubes on a jet are heated to, but it is far far hotter than 5°. Hot enough to melt your fingerprints off in a fraction of a second (literally... and I know from first hand experience... more than once....)$\endgroup$
– LnafzigerJan 29 '16 at 23:26

$\begingroup$Also, the "how can a heated pitot tube freeze" question is more of a physics question with an answer that would include something about rates of heat transfer and how many joules the heater produces -versus- the temperature and speed of the air/moisture that it comes into contact with.$\endgroup$
– LnafzigerJan 29 '16 at 23:35

$\begingroup$@Lnafziger: Thanks, corrected. We can heat a large windscreen, it would be interesting to know why it's difficult to heat a small tube. Jets are not short of joules. If there is no answer from the Aviation community, I'll move the question as suggested.$\endgroup$
– minsJan 30 '16 at 0:47

$\begingroup$According to this pdf, a smaller pitot tube submersed in ice water will still reach an internal temperature of 270° C. I can't imagine how hot they get when at a normal ambient temperature!$\endgroup$
– LnafzigerJan 31 '16 at 5:19

2 Answers
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Captain Bill Palmer, in his book "Understanding Air France 447" has a section dedicated to answering that very question as it pertains to AF447. Although there is no way to know for certain he puts forward some possibilities.

He quotes a commenter on a website:

One commenter on the Weather Graphics website’s AF447 article provided this interesting observation: “I'm an aircraft icing specialist and wanted to point out a factor that hasn't been discussed much … high ice crystal concentrations. I've seen flight test data from power rollbacks due to flight in high ice crystal environments … In our case, the crystals collected within heated, aspirated Ram Air Temperature sensors, forming a 0 ° C slush…”

He notes that just before the pilot tubes clogged, the sound of ice crystals hitting the windshield could be heard on the CVR.

Ice crystals bounce off the exterior of an airplane and cause no visible ice accretion, but they can enter the probe inlets. When highly specific climatic conditions exist in combination with certain combinations of altitude, temperature, and Mach, the concentration of ice crystals entering a probe can exceed its capacity to melt and evacuate the moisture through its drain holes. The result is that the ice crystals form a physical barrier within the probe that disrupts the measurement of total pressure.

The particular type of ice that may have been responsible was a substance called graupel.

Graupel forms when tiny supercooled water droplets adhere to snow crystals to the point that they engulf the snow crystal itself.

Graupel. Photo from Wikimedia commons

Capt. Palmer notes factors that make graupel a possible suspect:

No airframe icing. The supercooled water theory is discounted by the fact that the A330's icing detectors were not triggered.

Graupel has large enough particles to be audible on the voice recorder. It takes a particle with enough mass and inertia (a given density) to hit the fuselage with a sound, instead of flowing around it with the relative wind, like snow.

Graupel has enough mass to temporarily overwhelm pitot anti-icing when concentrations are high enough. The pitot tubes are hot. But even if you put a snowball on a hot skillet it does not melt instantaneously. If there is enough mass in the blockage, and in combination with new particles being added to the blockage as the first ones melt, it may exceed the pitot tubes capability to melt the obstruction as fast as it is introduced. Graupel is of significantly higher density than snow.

Graupel has sufficient blocking properties to prevent efficient transmission of dynamic pressure within the pitot tube.For example, water can flow and transmit pressure within the pitot tube, though it too can alter pitot-static readings, a physical non-fluid blockage could shield the pressure sensing port.

The likely presence of snow or similar form, as evidenced by the St. Elmo's fire discussed by the crew. The accident report stated that the sound of ice crystals hitting the aircraft can be heard about 20 seconds before the airspeed loss and autopilot disconnect.

It must be noted that the exact cause of icing problems on the A330 was never completely identified but it was specific to the particular brand of pitot tubes originally installed. Airbus was in the process of replacing them all with pilot tubes from a different manufacturer.

$\begingroup$So a possibility could be that the sudden arrival of the a large quantity of GS ice cannot be melt by the heat. Thanks for having named it. This calls indeed for another question: What would be the cost of improving probes to get rid of stubborn ice when heat is not adequate, rather than to stop providing measures for an undetermined amount of time. I'm suggesting a simple solution: Mount two opposite probes on a plate (active and backup) and rotate it when the active one stops working. It surely can't be a good solution, but other solutions must exist.$\endgroup$
– minsJan 30 '16 at 11:15

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$\begingroup$I don't know what the design differences are but Thales, the company that made the pitot tubes redesigned them and the newer model had fewer icing problems. They also had previous problems with BF Goodrich pitots and water ingestion. BFG redesigned these too. It is not completely avoidable but I don't think it is really a huge problem. The other times it happened it was transient and the pilots simply hand flew for a minute until it cleared. On AF447 it only lasted about 30 seconds. Just being aware that it can happen and knowing the correct reaction is the key.$\endgroup$
– TomMcWJan 30 '16 at 16:07

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$\begingroup$To my knowledge graupel was discounted in relation to AF447 a long time ago, it's too large for that. In the majority of cases graupel can also be detected by radar (assuming the crew were using it correctly). Ice crystal icing is a different beast, however.$\endgroup$
– os1Jan 30 '16 at 21:00

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$\begingroup$Probably not relevant to airliners, but I had a pitot icing incident on my Lancair that was interesting. The pitot heat was functioning correctly throughout. I had flown through a lot of rain early in the flight, and some liquid water had entered the pitot and gone past the heated portion of the probe and deeper into the lines connecting it to the ADAHRS. Later in the flight I climbed into colder air below 0 C. All was still well until I started my descent and pitched forward. This pitch angle caused the liquid water to run into a different, colder portion of the line and freeze, blocking it.$\endgroup$
– pericynthionFeb 3 '19 at 21:51

1) Pitot tubes are certified to withstand icing under particular circumstances: Within a given temperature range, precipitation amount, altitude etc. If they are operated outside of those ranges then the heating systems may not be effective. In the case of AF447 it was also important that they experienced ice crystal icing and not supercooled water icing - the two respond to pitot heating differently and the same certification requirements can't be used for both.

2) Automated systems are not, yet, perfect at detecting icing conditions. You don't want a system where the pitot is always on, with no possibility to turn it off, for safety reasons and you don't want an automatic system because it might fail to detect icing conditions and hence not activate the heat. That said, in my experience it's very rare to have the pitot heat off: There's not really much point. The crew has to have the discipline to turn it on at the beginning of the flight but after that an automatic system is not helpful if the heat is supposed to remain on anyway.

$\begingroup$According to the book I quoted in my answer, on the A 330 pilot heat is on anytime the engines are running. There's no switch$\endgroup$
– TomMcWJan 31 '16 at 2:29

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$\begingroup$TomMcW - they can be turned off, however - anything electrical on an aircraft has a circuit breaker$\endgroup$
– Jon StoryFeb 1 '16 at 15:53

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$\begingroup$"You don't want a system where the pitot is always on, with no possibility to turn it off, for safety reasons": I covered this point in the question (last bullet). A circuit breaker is necessary.$\endgroup$
– minsFeb 10 '16 at 8:17

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$\begingroup$"Pitot tubes are certified to withstand icing under particular circumstances": The question would be: Is the certification standard adequate? I understand that if this was easy, it would be done, but I don't understand clearly the difficulty.$\endgroup$
– minsFeb 10 '16 at 8:20